1. Field Of The Invention
This invention relates to an acid diffusion-dialysis membrane which may be used for effectively separating and recovering an acid from a mixture of an acid and a salt, or a mixture of an acid and a water-soluble organic compound, e.g., glucose, by dialysis non-electrolytically, and to methods of producing such membranes. More particularly, this invention relates to an acid diffusion-dialysis membrane having a high acid diffusion-dialysis coefficient and a high acid-salt selectivity ratio which is high in chemical stability and high in mechanical strength.
2. Description Of The Prior Art
Diffusion-dialysis techniques have been used for separating acids, such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, hydrofluoric acid, or the organic acids, e.g., acetic acid, from mixtures of acids and salts, such as mixtures resulting from smelting and refining processes for nickel metal, solvent extraction, etching processes, pickling processes or plating processes. It is also known that diffusion-dialysis can be used for separating acids from mixtures of acids and organic compounds such as from a cellulose saccharification solution, a lactam solution from a Beckmann rearrangement process, mixtures of acids and glucose, and mixtures of acids and amino acids.
As disclosed in Japanese Pat. No. 299,433, anion-exchange membranes have often been used as dialysis membranes for such separation and recovery. However, in the usual acid dialysis process, using an anion exchange membrane, an electric current is used to cause a transfer of anions. If dialysis is to be effected by means other than electrodialysis, i.e., depending only on the concentration differences between the solution to be separated and the recovery solution, the membrane used should have a high acid diffusion-dialysis coefficient and a high acid-to-salt or acid-to-organic compound selectivity ratio, to be suitable for industrial application.
Usually, in order to increase the acid diffusion-dialysis coefficient, the thickness of the membrane must be decreased. The thickness of the conventional acid diffusion-dialysis membrane, however, must be maintained at between several tens of microns to about one hundred microns, since thinner membranes suffer from severe pin-hole formation, cracking, and low mechanical strength.
Moreover, although the thinner the membrane, the higher will be the diffusion-dialysis coefficient, it has also been found that the thinner th membrane, the lower will be the degree of selectivity for separating acid-salt or acid-organic compound mixtures. For example, one conventional diffusion-dialysis membrane prepared by coating a specific epoxy resin onto a reinforced fabric (U.S. Pat. No. 3,152,061), the relationship between acid diffusion-dialysis coefficient and thickness of membrane, and the relation between selectivity ratio for separating acid from salt and thickness of membrane are, respectively, shown in FIG. 1. In other words, in most conventional acid diffusion-dialysis membranes, as the thickness of the membrane decreases, the acid diffusion-dialysis coefficient increases, but the selectivity ratio significantly decreases.
It has been proposed to cross-link the membrane to a high degree in order to maintain a high selectivity ratio in thin membranes. However, as the density of cross-linkages increases, the acid diffusion-dialysis coefficient adversely decreases. It has been quite difficult, therefore, to increase both the acid diffusion-dialysis coefficient and the acid-salt or acid-organic compound selectivity ratio.